$OpenBSD: patch-CMakeLists_txt,v 1.2 2017/05/13 21:56:14 espie Exp $
set C++ options explicitly, clang complains for C code.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -64,7 +64,8 @@ elseif(WIN32)
 endif()
 
 if(CMAKE_COMPILER_IS_GNUCXX OR MINGW OR ${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")
-    add_compile_options(-std=c++11 -W -Wall -O3 -pedantic)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+    add_compile_options(-W -Wall -pedantic)
     add_compile_options(-fomit-frame-pointer -finline-functions -ffast-math)
 elseif(MSVC)
     add_definitions(-DNOMINMAX)
